This invention relates generally to digital communication systems and, particularly, to such systems which provide for multiple access to a plurality of signals carried on a single communications medium. More specifically, this invention relates to telephone extension systems, by which signals are transferred simultaneously between a plurality of telephone lines and telephone extensions by means of the AC power lines of a building or an RF (radio frequency) transmission medium. This invention also relates to secure system control protocols for line carrier telephone extension systems, which provide multiple extension arbitration logic, privacy, security and other telephone signaling options, which technique is called Secure Access (SA). The inventive method may further be implemented in a cordless telephone system and may be combined with caller I.D. technology.